Frequency modulation receiver intensity indicator



Zer

N 9 1945' P. F. G. HOLST ET AL 2,388,544

FREQUENCY MODULATION RECEIVER INTENSITY INDICATOR Filed Dec. 2, 1941 INVENTOR$ ATTORNEY Patented Nov. 6, 1945 FREQUENCY MODULATION RECEIVER INTENSITY INDICATOR Paul F. o. Holst an Loren R. Kirkwood, Oaklyn, N. J., assignors to Radio Corporation of America, a. corporation of Delaware Application December 2,1941, Serial No. 421,236

1 Claim.

Our present invention relates to signal strength indicators for receivers of angular velocity-modulated carrier waves, and more particularly to a signal intensity indicator for frequency modulated carrier wave receivers which will indicate the strength of the signal carrier to which the receiver is tuned.

In amplitude-modulated carrier wave (AM) broadcast receiversit has often been found advantageous to utilize a diode as the second detector. The diode current has been used for automatic volume control action of the receiver. In such case, the diode current hasoften been employed to indicate the strength of the incoming carrier signal. Furthermore, in such amplitude modulation receivers, regardless of the construction of the automatic volume control circuit, it is readily possible to provide a signal strength indicator by merely having'it respond to the direct currents, or potentials, produced by the second detector. However, in the case of receivers of angular velocity-modulated carrier waves, such as frequency-modulated carrier waves, automatic volume-control, as known in AM receivers, is rarely employed. Furthermore,

the usual second detector device is replaced by a so-called discriminator whose total output voltage is zero at resonance.

One of the main objects of our present invention is to provide in a receiver of angular velocitymodulated carrier waves a plurality of cascaded to provide a frequency modulated carrier wave (FM) receiver which has a plurality of cascaded intermediate frequency (IF) amplifier stages simultaneously functioning asamplitude modulation limiters, avisual current indicator being operatively associated with each of the IF stages,

and means being provided for selectively connecting the visual indicator with the desired one of the IF stages thereby'to permit indication of signal intensity over a wide intensity range.

Still another objectof our invention is to improve generally the simplicity and efiiciency of signal strengthindicators for receivers of angular velocity-modulated carrier waves, and more especially to provide an FM receiver which includes a signal intensity indication means which is not only reliable and eficient in operation, but is economical of manufacture and assembly.

The novel features which we believe to be characteristic of our invention are set forth with particularity in the appended claim; the invention itself, however, as to both its organization and method of operation will best be understood by reference to the following description taken in connection with the drawing in which we have indicated diagrammatically a circuit organization whereby our invention may be carried into effect.

In the drawing: v

Figure 1 shows a circuit diagram of th invention; v

Figure 1A shows a modified embodiment of the indication meter switching mechanism;

Figure 2 graphically illustrates the operation of the indicating device.

Referring, now, to the accompanying drawing, wherein like reference characters inthe different figures designate similar circuit elements, we have shown in Figure 1 only that portion of an FM receiver which is essential to a proper understanding of our present invention. Let it be assumed that the IF amplifier network shown herein is, by way of illustration, the IF amplifier network of the FM receiving system disclosed in our application Serial No. 421,900 filed December 6,

1941 patented January 16, 1945 as U. S. Patent No. 2,367,352. In that receiving system, the desired signal frequency band covers a range of 70.4 to 72.6 megacycles (mc.). The IF value, that is the center frequency of the FM waves reduced to the IF, is 5 me. The channel width of the received FM waves may be of the order of kilocycles kc.) Of course, the invention is not limited to these various frequency values, since the carrier frequency deviation ratio may be small or high depending upon the desires of the trans mitter and receiver set designer.

Assuming, however, that each of the IF intertube networks has a pass band width of 150 kc, the IF transformer I will have its primary and secondary circuits each tuned to Fe, which, in this case, would be the operating IF value (5 mo). The primary and secondary circuits would be coupled to pass the entire frequency swing of the IF carrier. It will be understood that the primary circuit of the transformer l is connected in the output circuit of the first detector, or converter, stage of an FM superheterodyne receiver. It is, also, to be clearly understood that, while reference is made herein to FM reception, the system is equally useful in connection with the reception of phase-modulated carrier Waves. The generic term angular velocity-modulated carrier waves" is used herein to include both of these forms of modulation.

Each or the amplifier tubes may be pentodes of the GSH'T; type. The: IF amplifier tubes; 2; 3 and 4 are arranged in cascade, and' the circuits thereof are substantially similar in construction. Therefore, a specific description will be given only in connection with one of; the IE amplif er stages, it being understood that each or; the other IF amplifier stages are exactly the same in construction.

Referring, then, to the IF amplifier tube 2;;its, cathode is at ground potential. Its input grid 5 is connected to the high alternating potential side of the secondary circuit of transformer 1 through the IF coupling condenser 6. potent al e of, the secondarycir uit. r transformer- I; is grounded. The grid side of thecondenser 6 is connected to a contactpoingtl throu h a pa wh h inc ud s: the se ies esis o t and, 8.. The. junction of. the resis ors. l and 8 is o nec to. ground. thr ugh a condens r 9, Resistors 1 and: 8, may-have magnitudes. of, 0.22 m so m an 1,000; ohms, respectively-L The plate or the tube 2: is; cQnnected to a source of positive, potential through the; coil of; the primary circuit or the IF tra form r ML In the second amplifier stage, the tube 3 its; control grid; connected. to,- contact. point II through the series resistors 7' and; it. The control g idis c pled to. he h h. alternat ng. potential side of the resonant input circuit of tube 3 through th co p c ndenser 6/. S milarl e h dIE mn ifi rtube 4; has its. ontro r connected tocontact point III through the: series resistors 1" and 8". The coupling'cqr dehser 6 connects. the c n rol grid of tub o t e h alternating pot nt sid or the esonant nput circuit of: ube 4- It will be clearly understood tha each of; the

primary and; secondary circuits of transformers. I,

In, H and I2 is. tunedto the operating IE: v lue, and: tha each of. the t nsformers is e igned, to pasS- a. hand: of" aPProXimately 15,0. kc, in width- The secondary circuit, of; transformer 1,2 may be pled: to, an FM demodulator network of any desired type, as, ror example, that shown in our af r sa app ation. d m dulated si nals will, of course, be: amplified, and then. reproduced in any suitable reproducer.

The signal strength indicating device ma be a current. meter which is schematically represented bynumber 1-3,. Qneof the terminals of the meter is established at grouhdpotentiahwhi e the other terminal is; connected toadjustable, or mobile, arm I142, It; w lzbe underst od: h t i justable element, 14: may; be connected with any of the contacts I, II. or A; second adjustable device is provided, and. comprises: a, plurality of arms l5, Hi, i! and [85. Itwill be understood that the spacing between these arms I5, i5, H and I-B maintained fixed. In other words; these four arms are moved as; a common unit, They cooperate with contact points I55 t6." and 1:1- The junction of the. four arms. 15; to- I82 inclusive is maintained at ground potential; Furthermore, thedotted line H designates a mechanical; un control mechanism which may be arranged for operating the unit {5mm concurrently with the adjustable arm 14-.

The lower end of resistor 8 i'sconnected to. contact l5. The lower end of resistor 8 is con- The low nected to contact l6; and the lower end of resistor 8 is connected to contact l1. It will, therefore, be seen as shown in Figure 1 that when the arm I4 is in electrical contact with point I11, then the arms l5 and I6 are in electrical contact with points l5' and I6 respectively. In other words, the grid resistorsofi amplifier'ti h s 2 and 3 are in such case connected directly to ground, whereas the grid resistors of amplifier 4 are connected to ground through the current meter l3.

Qf QQH'IPSQ mflaHS that current meter l3 will indicategonly the grid current flowing through the grid circuit of amplifier tube 4. Similarly, when mechanism L9 is actuated to adjust arm l4 to make, electrical contact with point II, the arm IE will make electrical contact with point [5. The arm I11; willmake electrical contact with point l'l'., It will, therefore, be seen that in this position of the switching mechanism I9 the meter I3 is circuit t indicate the gr durrent low!- in th r d c rcu r ub 3, wher asthe: gr d ircuits oi tubes 2 a d 4 re g ounde Ihe ascaded sta es including ubes, 2, 3 and 4 provide n AM imiter network, Br e th func ionf. t l m t to elhnnnatov AM. ef ects which may exist on the FM can-t er and WhiQh e fe t are aused y virtue r n ise m uls s, fa g, and; a so. by virtue of the passage; r h signal waves through they cascadedresonant; oi cui prior to h dem du a or; im rs a s f nct on as an AVG (automa c v hune control dev e, limit rs; used; n HM receivers ar often of: the r d. ee-h 1im 'tm2e;; that. s the, amplifi tu whi h usual y an ampl n ri s f; a adi y over oaded typ -v Eorexample, he cathode o the amplifie s; rou ded, wh le the control g d s c n c ed to h innut, c r it; b a rid condense e a e ng; c nne t d t round t rough. a. g dls esist r.

As: the input, vo ta e t suoha g st l ek re f limit sra ed. t e, direct currentvol ge en. the contr gr d f: th amplifie ube increases an the e y automat c ly tends to ower h outpu current of; th ube- Bx proper d ign ithe, associated circuits, it is possible to, maintai the output subs a tia y con ant when the signa input voltage is raised about; a, Qcrtai n. minimum value. l lcnwev .r, the; grid current flowin thro e gr eak resistor. of the. limiter tube increasesa with increasing signal. strength, In t e words, a th ugh th inpuho tout harmteristic has a rising portion upto; a. predetermined.- carrier amplitude, and. a, substantially horizontal; portion thereafter, yet the grid, current flowthrough the grid leak resistor keeps-increasing above the atoresaid predetermined carrier amplitude value. This. increase is, maintained until grid current reaches avalue at, which th pr ced g a ifi r t be b gins, to. limit- It, has been found that a g od. desi n r. a. limiter as: described will maintam the, output. con.-

stant over a range. of signal inp t. 01,- approximately 40-c1b (decibels).. Inorder topermit, int r ations ov r a r ater range. or. signal in.- Put voltage, all the stages, in the.- IE amplifier n work. ar made to, un tion as l miters In other words, the grid condenser-grid leak resistor combination, 6*! of each of the; IE amplifier sta s fun tions to pr id AM lim ting a t o at di ferent va es o s g al nput. voltage... It will now be seen that, the, entire, network between th convert r utput cir uit. and the.- deniedulatorfunctions as an: limiter, the limitin action be g s ag ered. Simultaneonslsi these limiter stages function as IF amplifiers. The

current meter I3 is selectively inserted in either of the three IF amplifier grid circuits, and the grid circuits which are not utilized are short-circuited to ground in order to permit them to function properly. This enables the set operator to measure at will the grid current flow in each of the grid circuits. By way of illustration, condensers 6, 6' and 6" may each have a value or 33 mmf. (micromicrofarads), while each of resistors 1, 1' and 'I" may have a value of 220,000 ohms.

In Figure 2, there is shown the relation between micro-volt input at the antenna of the receiving system, and grid current flow in each of the IF amplifier-limiter stages. The curves denoted as I, II and III indicate the grid current flow through the current meter when the arm l4 is adjusted for contact with the contact points I, II and III, respectively. It will be seen that the strength of signal applied to the antenna input terminal of the receiver may be calibrated directly against the grid current flow in each of the IF amplifier stages. The resistors 8, 8' and 8", in conjunction with each of the condensers 9 8' and 3", respectively, function as IF filter networks.

It is believed that the operation of the mechanism will be clear from the aforesaid description. It will be noted that grid current flows in th third IF amplifier stage for smaller microvol-t input values than in the first IF amplifier stage. Hence, when the switching mechanism is actuated to indicate the grid current flow in the grid circuit of the third IF amplifier tube 4, that will be the adjustment for indicating weak signal strength. On the other hand, when the current-indicating device is in circuit solely with the grid circuit of the first IF amplifier tube 2, then it will be able to provide an indication of a relatively high field strength. Curve III of Figure 2 indicates a weak signal intensity, while curve I indicates a strong signal.

In Figure 1A, there is shown a simplified mechanical switch mechanism which can be utilized in place of the three position switching mechanism shown in Figure I. The simplified switching mechanism comprises a split ring device of sections 30 and 3|. The indicating meter I3 is connected between ground and one wiper, or contact, element 32. The contact points I, II

and III are connected respectively to wiper elements 33, 34 and 35. An auxiliary wiper element 34 is also connected to contact point II. This wiper acts to connect/the grid circuit of ampliher 3 to ground in the extreme clockwise position of 3l-30. In addition, a grounded wiper element 40 is provided. The wipers are all resilient. It will now be seen that, when the rotatable switching mechanism 3I-30 is in the position shown (extreme counter-clockwise), the indicating meter is in circuit with the grid circuit of the third IE amplifier tube.

It will be noted that the grid circuits of the first two amplifier tubes are connected to ground by virtue of the fact that the wiper elements 40, 33 and 34 are in electrical connection with the metallic ring section 3|. It will be clear that proper rotation of the switching ring will connect the indicating device I 3 in circuit with either of the "grid circuits of the first or second IE amplifiers, while connecting the grid circuits of the non-selected amplifiers to ground.

While we have indicated and described a system for carrying our invention into effect, it will be apparent to one skilled in the art that our invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of our invention, as set forth in the appended claim.

What we claim is:

In combination with a source of frequency modulated carrier waves, a plurality of amplifier tubes arranged in cascade, each of said amplifier tubes including a grid condenser-leak resister network for providing limiting therein. a current indication means, a first means for selectively connecting said current indication means to a desired one of said grid leak resistors thereby to indicate grid current flow therethrough, a second means connecting the grid leak resistors of the remaining amplifier tubes to a point of relatively fixed potential thereby to prevent grid current flow in said remaining amplifier tubes from affecting said current indication device and a common actuating means for concurrently operating said first and second means.

PAUL. F. G. HOLST. LOREN R. KIRKWOOD. 

